Products with variability in multiple attributes, such as contact lenses, require supply sets with a large number of the variable attribute combinations ready and available to a potential customer to be most effective for the product manufacturer. If the correct combination of attributes is not available to the potential customer at the right time, there is a potential loss in sales, as the customer may turn to another brand that can meet their needs more immediately. For example, contact lenses come in a variety of types, such as hard and soft contacts; a variety of prescription powers; and other attributes (like cylinder power and axis measurements for toric lenses), just to name a few.
Often, a contact lens manufacturer will supply trial fit cases to optometrists and ophthalmologists that contain sample contact lenses that meet a wide range of these attribute combinations. With these fit cases, the doctor can select contacts with the proper prescription combination and let the patient try them immediately. However, patients are not typically willing to wait for their doctor to order contacts simply for a trial set that may or may not work. As such, if the doctor finds that a particular contact prescription is empty in a fit case, he or she is likely to just turn to another manufacturer's fit case. This will often result in a lost sale for the initial contact manufacturer, as patients are likely to proceed with the brand that they try. The patient may inherently believe that the selection is a recommendation of brand from their doctor, rather than what may simply be the result of poor supply in the test kit.